The invention relates to a method for producing a selective doping structure on a semiconductor substrate in order to produce a photovoltaic solar cell.
A photovoltaic solar cell is a planar semiconductor element in which generation of electron-hole pairs is achieved by means of incident electromagnetic radiation and charge carrier separation is effected at at least one pn junction, such that an electrical potential difference arises between at least two electrical contacts of the solar cell and electrical power can be tapped off from the solar cell via an external electrical circuit connected to said contacts.
Typical solar cells comprise a semiconductor substrate having a base doping, wherein at least one emitter region having an emitter doping, which is opposite to the base doping, is formed in the semiconductor substrate, such that the abovementioned pn junction forms between base region and emitter region. Contact is made with base region and emitter region in each case by at least one metallic contact structure for collecting and carrying away the charge carriers.
In order to achieve high efficiencies, an optimization has to be effected with regard to a number of loss mechanisms: thus, a high doping is advantageous for forming a low electrical contact resistance between metallization and contact-connected semiconductor region. On the other hand, a higher doping leads, in principle, to a higher recombination of electron-hole pairs within the semiconductor substrate.
It is therefore known, both in the emitter region and in the base region, to form selective doping structures.
Thus, the formation of selective emitter structures is known, for example, in which, for example, on the front side of the semiconductor substrate facing the light, an emitter having a first doping profile is formed in a planar fashion and a second emitter profile having a higher doping than the first profile is formed only in the regions in which contact is intended to be made with the emitter by a metallic emitter contact structure applied to the front side of the semiconductor structure. Such a selective emitter ensures that, in the case of transverse conduction of the charge carriers within the emitter region of the first emitter profile, a reduction of the recombination is achieved and, on the other hand, a low contact resistance with respect to the metallic contact structure is achieved on account of the higher doping of the second doping profile.
Various methods have been proposed for industrially implementing the production of selective emitters: thus, Haverkamp, H., et al. Minimizing the Electrical Losses on the Front Side: Development of a Selective Emitter Process from a Single Diffusion. in 33rd IEEE PVSC. 2008. San Diego, describes the production of a selective emitter by means of homogenous diffusion of an emitter region, subsequent masking and wet-chemical, partial etching-back of the semiconductor substrate. Furthermore, Bultmann, J. H., et al. Single Step selective emitter using diffusion barriers. in 16th EU PVSEC.2000. Glasgow, describes the production of a selective emitter structure by applying and patterning a semitransparent diffusion barrier. Furthermore, Jäger, U., et al. Selective emitter by laser doping from phosphosilicate glass. in Proceedings of the 24th European Photovoltaic Solar Energy Conference. 2009. Hamburg, Germany, describes the production of a selective emitter structure by selective laser doping.
The abovementioned methods require in some instances complex and cost-intensive masking steps or lead to emitter profiles which, on account of the profile progression, still significantly exhibit recombination losses and thus reductions of efficiency.